The present application is related to the co-pending application of David L. Davies, entitled "TUBING PRESSURIZED FIRING APPARATUS FOR A TUBING CONVEYED PERFORATING GUN", Ser. No. 518,910 Filed Aug. 1, 1983, and assigned to the assignee of the present invention.
1. Field of the Invention
The present invention relates generally to firing devices of the type used to fire perforating guns used to perforate a cased well bore for the production of well bore fluids and, specifically, to a tubing pressurized firing device for firing a tubing conveyed perforating gun responsive to changes in the fluid pressure within the well bore tubing string.
2. Description of the Prior Art
The integrity of oil well and gas well bores is generally preserved during drilling operations by cementing a casing or liner in place in the bore hole. The casing or liner is a cylindrical conduit which must be punctured or perforated over the desired production interval in order to produce well bore fluids. Various types of perforating guns are known which utilize some form of a fired projectile and an explosive charge to perforate the casing or liner. Prior perforating gun techniques have either utilized tools which were run on a wireline or cable or were tubing conveyed devices which were run on a tubing string to the desired depth in the well bore.
Several problems exist with wireline run perforating gun systems. Wireline methods require a delicate balance between expected formation pressure and the drilling mud or fluid used to provide a balanced hydrostatic head in the well bore. A miscalculation of the expected formation pressure can result in a tangled wireline. If an over balanced pressure condition exists during the perforation step, the well can loose large volumes of fluid into the surrounding formation which can damage the formation. Also, proper pressure differentials are needed to effectively clean the perforations. It is difficult with the wireline system to obtain the pressure differential needed to back surge the perforations and provide a clean well which will produce high yields. It is also difficult to use wireline systems in deviated well bores.
The tubing conveyed perforating gun generally allows immediate safe release of formation pressure at maximum pressure differentials into the tubing string. The back surge which results tends to clean the perforation of mud filtrate, cement, and other perforating debris. In spite of these advantages, prior tubing conveyed perforating guns have lacked the ideal detonating system. Prior systems were usually detonated by (1) a bar dropped through the tubing string to fire a percussion detonator; (2) a through-the-tubing wireline connection system using an electrical charge to detonate the gun; or (3) well annulus pressurized systems which were actuated by fluid pressure acting through ports in the tubing string located above the packer. The first two systems require a wireline run in order to set a hydraulic packer to seal off the well bore. The third system above could only be used with a mechanically set well packer or a previously set permanent packer. Also, there was no satisfactory delay mechanism for the detonator of the gun.
It is an object of the present invention to provide a tubing conveyed perforating gun which is actuated by pressuring the tubing interior from the well surface.
Another object of the invention is to provide a tubing conveyed perforating gun which can be utilized with a hydraulically set packer where the packer is set by pressuring the tubing to a first predetermined level and where the perforating gun is actuated by pressuring the tubing to a second predetermined level.
Another object of the invention is to provide a tubing conveyed perforating gun with a delay mechanism which provides a known delay interval between pressuring the tubing to the second predetermined level and the actual firing of the perforating gun.